The present invention relates to heterocyclic monomers, particularly to o-amino-hydroxy-benzothiazole-benzoic acid and a new enzymatic process to synthesize it from the corresponding nitro-benzothiazole-benzoic acid compound.
4,6-Diaminoresorcinol and 2,5-diamino-1,4-benzenedithiol (usually stored as dihydrochloride salts) are key co-monomers for the synthesis of high-strength-high-modulus, thermally resistant rigid-rod poly(p-phenylenebenzobisoxazole) (PBO) and poly(p-phenylenebenzobisthiazole (PBT) polymers for lightweight structural, nonlinear optical and electronic applications.

More recently, PBO and PBT have been considered for use in the proton-exchange membranes of fuel cells. While an AB-monomer 4-[5-amino-6-hydroxybenzoxazol-2-yl]benzoic acid has been prepared and utilized in the synthesis of the correspond PBO polymer, the analogous AB-monomer for PBT, namely, 4-[5-amino-6-mercaptobenzothiazol-2-yl]benzoic acid is yet to be synthesized. In addition to having an intrinsically perfect stoichiometry that helps to promote high molecular weight polymers in polycondensation processes, AB-monomers are also useful starting materials for the synthesis of AB diblock copolymers, ABA triblock copolymers and star polymers. They can also be grafted onto appropriately functionalized surfaces. The availability of the above aminophenol derivative is limited because the synthesis is complex and yields are low. Recent advances in biologically converting aromatic nitro compounds to the corresponding o-aminophenols suggest the possibility of synthesizing novel AB-monomers such as 4-(6-amino-5-hydroxybenzothiazol-2-yl)benzoic acid, and in principle, the corresponding rigid-rod polymer that is a hybrid of both PBO and PBT with respect to the chemical structure.
Accordingly, it is an object of the present invention to provide a process for the preparation of the 4-(6-amino-5-hydroxybenzothiazol-2-yl)benzoic acid.
Although the requisite starting nitro compounds (either as a carboxylic acid, viz. 2-(4-carboxyphenyl)-6-nitrobenzothiazole, or an ethylester, viz. 2-(4-carboethoxyphenyl)-6-nitrobenzothiazole) are relatively simple molecules, they have not been reported in open or patent literature. Thus, it is another object of this invention to provide the following nitrobenzothiazolecarboxylic acid.

The simple route to the subject nitrobenzothiazolecarboxylic acid is via direct nitration of 2(4-carboxyphenyl)benzothiazole, which can be synthesized from the condensation reaction between 2-aminothiophenol and 4-carboxybenzaldehyde in hot dimethyl sulfoxide. However, the poor solubility of 2(4-carboxyphenyl)benzothiazole and 2-(4-carboxyphenyl)-6-nitrobenzothiazole has necessitated additional esterification/de-esterification steps in order to rigorously establish the identity and purity of the subject AB-monomer.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.